Paste composition for solar cell electrodes and electrode fabricated using the same

ABSTRACT

A paste composition for a solar cell electrode includes including an organic vehicle, a conductive powder, and a glass frit, the glass frit including TeO 2 , and a transition metal oxide component, the transitional metal oxide component including one or more of a transition metal oxide having a melting point of about 1300° C. or more.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. §119 to KoreanPatent Application No. 10-2012-0127763, filed on Nov. 12, 2012, in theKorean Intellectual Property Office, and entitled: “Paste Compositionfor Solar Cell Electrodes and Electrode Fabricated Using the Same,”which is incorporated by reference herein in its entirety.

1. Field

Embodiments relate to a paste composition for solar cell electrodes andelectrodes fabricated using the same.

2. Description of Related Art

Solar cells may be used to generate electric energy through thephotovoltaic effect of a p-n junction that converts photons of sunlightinto electricity. In the solar cell, a front electrode and a rearelectrode may be respectively formed on upper and lower surfaces of asubstrate, e.g., a semiconductor wafer, etc., with the p-n junction. Thephotovoltaic effect of the p-n junction may be induced by sunlightentering the semiconductor wafer and electrons generated by thephotovoltaic effect of the p-n junction provide electric current flowingto the outside through the electrodes.

SUMMARY

Embodiments are directed to a paste composition for a solar cellelectrode, the composition including an organic vehicle, a conductivepowder, and a glass frit, the glass fit including TeO₂, and a transitionmetal oxide component, the transitional metal oxide component includingone or more of a transition metal oxide having a melting point of about1300° C. or more.

The glass fit may include about 1 wt % to about 15 wt % of thetransition metal oxide component.

The transition metal oxide may include at least one of NiO, WO₃ andCo₂O₃.

A weight ratio of the TeO₂ to the transition metal oxide component mayrange from about 2:1 to about 5:1.

The glass frit may include about 15 wt % to about 70 wt % of the TeO₂.

The TeO₂ and the transition metal oxide component may be present in atotal amount of about 16 wt % to about 75 wt % in the glass fit.

The glass fit may further include about 5 wt % to about 35 wt % of Bi₂O₃and about 10 wt % to about 50 wt % of PbO.

The glass frit may further include about 1 wt % to about 20 wt % of ZnO.

The glass frit may further include one or more of Al₂O₃, ZrO₂, P₂O₅,SiO₂, Na₂O, B₂O₃, Ta₂O₅, Fe₂O₃, Cr₂O₃, CO₂O₃, Li₂O, Li₂CO₃, MgO, orMnO₂.

The conductive powder may include one or more of silver, gold,palladium, platinum, copper, chromium, cobalt, aluminum, tin, lead,zinc, iron, iridium, osmium, rhodium, tungsten, molybdenum, nickel, orindium tin oxide powder.

The organic vehicle may include one or more of a binder or a solvent.

The composition may include about 5 wt % to about 30 wt % of the organicvehicle, about 60 wt % to about 90 wt % of the conductive powder, andabout 1 wt % to about 10 wt % of the glass frit.

The composition may further include one or more of a dispersant, athixotropic agent, a plasticizer, a viscosity stabilizer, ananti-foaming agent, a pigment, a UV stabilizer, an antioxidant, or acoupling agent.

Embodiments are also directed to a solar cell electrode fabricated usingthe composition according to an embodiment.

Embodiments are also directed to a method of fabricating a solar cellhaving a solar cell electrode, the method including printing thecomposition according to an embodiment on a substrate, and baking thesubstrate having the composition thereon to form the solar cellelectrode.

BRIEF DESCRIPTION OF DRAWINGS

Features will become apparent to those of skill in the art by describingin detail example embodiments with reference to the attached drawing inwhich:

FIG. 1 illustrates a schematic view of a solar cell manufactured using apaste composition in accordance with an embodiment.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawing; however, they may be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey example implementations to those skilled in the art. In thedrawing figures, the dimensions of layers and regions may be exaggeratedfor clarity of illustration. Like reference numerals refer to likeelements throughout.

An example embodiment relates to a paste composition for solar cellelectrodes, which includes a conductive powder, a glass frit, and anorganic vehicle.

In an example embodiment, the composition may include about 60 wt % toabout 90 wt % of the conductive powder, about 1 wt % to about 10 wt % ofthe glass fit, and about 5 wt % to about 30 wt % of the organic vehicle.

Conductive Powder

Examples of the conductive powder may include silver (Ag), gold (Au),palladium (Pd), platinum (Pt), copper (Cu), chromium (Cr), cobalt (Co),aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), iron (Fe), iridium (Ir),osmium (Os), rhodium (Rh), tungsten (W), molybdenum (Mo), nickel (Ni),and magnesium (Mg) powder, etc. These conductive powders may be usedalone or as a mixture or alloy of two or more thereof. The conductivepowder may include silver powder. In some example embodiments, theconductive powder may further include nickel (Ni), cobalt (Co), iron(Fe), zinc (Zn), or copper (Cu) powder in addition to the silver powder.In some example embodiments, the conductive powder may include indiumtin oxide (ITO) powder.

The conductive powder may have a spherical, flake, or amorphous particleshape.

The conductive powder may be a mixture of conductive powders havingdifferent particle shapes.

The conductive powder may have an average particle size D50 of about 0.1μm to about 3 μm. The average particle size may be measured using, forexample, a Model 1064D particle size analyzer (CILAS Co., Ltd.) afterdispersing the conductive powder in isopropyl alcohol (IPA) at 25° C.for 3 minutes via ultrasonication. Within this range of average particlesize, the paste composition may provide low contact resistance and lineresistance. In an example embodiment, the conductive powder may have anaverage particle size (D50) of about 0.5 μm to about 2 μm.

The conductive powder may be a mixture of conductive particles havingdifferent average particle sizes (D50).

The conductive powder may be present in an amount of about 60 wt % toabout 90 wt % in the paste composition. Within this range, theconductive powder may prevent deterioration in conversion efficiency ofa solar cell due to resistance increase and difficulty in forming thepaste due to relative reduction in amount of the organic vehicle. In anexample embodiment, the conductive powder may be present in an amount ofabout 70 wt % to about 88 wt %.

Glass Frit

The glass frit may include TeO₂. The glass frit may include a transitionmetal oxide component. The glass frit may provide low contact resistanceand high junction quality. The transition metal oxide may have a meltingpoint of about 1300° C. or more.

In the glass frit, the TeO₂ may be present in an amount of about 15 wt %to about 70 wt %, e.g., about 20 wt % to about 40 wt % or about 20 wt %to about 35 wt %. Within this range, the paste composition may provideexcellent properties in terms of contact resistance.

The transition metal oxide may have a melting point ranging from about1300° C. to about 2000° C. For example, the transition metal oxidehaving a melting point of about 1300° C. or more may include one or moreof NiO, WO₃ and Co₂O₃. The transition metal oxide component may bepresent in an amount of about 1 wt % to about 15 wt %, e.g., about 3 wt% to about 12 wt % or about 5 wt % to about 10 wt % in the glass frit.Within this range of the transition metal oxide component, the pastecomposition may minimize adverse influence on the p-n junction andreduce contact resistance.

In an embodiment, the TeO₂ and the transition metal oxide component maybe present in a total amount of about 16 wt % to about 75 wt %, e.g.,about 20 wt % to about 60 wt % or about 25 wt % to about 50 wt %, in thefrit. Within this range, the paste composition may exhibit excellentproperties in terms of contact resistance.

In an example embodiment, the weight ratio of TeO₂ to the transitionmetal oxide component may range from about 2:1 to about 5:1. Within thisrange, the paste composition may minimize adverse influence on the p-njunction and reduce contact resistance.

In an example embodiment, the glass frit may further include about 5 wt% to about 35 wt % of Bi2O3 and about 10 wt % to about 50 wt % of PbO.

In some example embodiments, the glass frit may further include about 1wt % to about 20 wt % of ZnO. Within this range, the glass fit mayprovide further enhanced efficiency.

In an example embodiment, the glass frit may further include one or moreof Al₂O₃, ZrO₂, P₂O₅, SiO₂, Na₂O, B₂O₃, Ta₂O₅, Fe₂O₃, Cr₂O₃, Co₂O₃,Li₂O, Li₂CO₃, MgO, or MnO₂. The composition of the respective componentscontained in the glass fit may be adjusted in consideration ofefficiency of the electrode or stability at high temperature.

The glass frit may be a crystallized glass frit or a non-crystallizedglass frit. Further, the glass frit may be any of a leaded glass frit, alead-free glass frit, and mixtures thereof.

The glass frit may be prepared by mixing metal oxides and the like inthe above amounts using a typical method. Mixing may be performed using,e.g., a ball mill or a planetary mill. In an example embodiment, themixed composition is melted at about 900° C. to about 1300° C., followedby quenching at about 20° C. to about 30° C. The resultant may besubjected to pulverizing using a disk mill or planetary mill to preparethe glass frit.

The glass fit may have an average particle size D50 of about 0.1 μm toabout 5 μm, e.g., about 0.5 μm to about 3 μm. The average particle sizeD50 may be measured using, for example, a Model 1064D particle sizeanalyzer (CILAS Co., Ltd.) after dispersing the conductive powder inisopropyl alcohol (IPA) at 25° C. for 3 minutes via ultrasonication.

The glass frit may be present in an amount of about 1 wt % to about 10wt % in the paste composition. Within this range, it may be possible toimprove sintering properties and adhesion of the conductive powder whilepreventing deterioration in conversion efficiency due to resistanceincrease. Further, it may be possible to prevent an excess of the glassfrit from remaining after baking, which may cause resistance increaseand wettability deterioration. In an example embodiment, the glass fritmay be present in an amount of about 1 wt % to about 7 wt % in the pastecomposition.

Organic Vehicle

The organic vehicle may include an organic binder that providesliquidity to the paste.

Examples of the organic binder may include cellulose polymers, such asethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose,hydroxyethyl hydroxypropyl cellulose, and the like; acrylic copolymersobtained by copolymerization with hydrophilic acrylic monomers such ascarboxyl groups; and polyvinyl resins, etc. These binders may be may beused alone or in combinations thereof.

The organic vehicle may include a solvent. For example, the organicvehicle may be a solution prepared by dissolving the organic binder inthe solvent. The organic vehicle may include about 5 wt % to about 40 wt% of the organic binder and about 60 wt % to about 95 wt % of thesolvent. In an example embodiment, the organic vehicle may include about6 wt % to about 30 wt % of the organic binder and about 70 wt % to about94 wt % of the solvent.

The solvent may be an organic solvent having a boiling point of 120° C.or more. The solvent may include carbitol solvents, aliphatic alcohols,ester solvents, cellosolve solvents, hydrocarbon solvents, etc., whichare commonly used in the production of electrodes. Examples of solventssuitable for use in the paste composition may include butyl carbitol,butyl carbitol acetate, methyl cellosolve, ethyl cellosolve, butylcellosolve, aliphatic alcohols, terpineol, ethylene glycol, ethyleneglycol monobutyl ether, butyl cellosolve acetate, texanol, or mixturesthereof.

The organic vehicle may be present in an amount of about 5 wt % to about30 wt % in the paste composition. Within this range, it may be possibleto provide efficient dispersion while avoiding an excessive increase inviscosity after preparation of the paste composition, which may lead toprinting difficulty, and to prevent resistance increase and otherproblems that may occur during the baking process. In an exampleembodiment, the organic vehicle may be present in an amount of about 5wt % to about 15 wt %.

In some embodiments, the paste composition may further include additivesto, e.g., enhance flow properties, process properties, and stability.The additives may include dispersants, thixotropic agents, plasticizers,viscosity stabilizers, anti-foaming agents, pigments, UV stabilizers,antioxidants, coupling agents, etc. These additives may be used alone oras mixtures thereof. These additives may be present in an amount of,e.g., about 0.1 wt % to about 5 wt % in the paste composition.

Another example embodiment relates to an electrode formed of the pastecomposition for solar cell electrodes and a solar cell including thesame. FIG. 1 shows a solar cell in accordance with an exampleembodiment.

Referring to FIG. 1, a rear electrode 210 and a front electrode 230 maybe formed by printing and baking the paste composition according to anembodiment on a substrate, e.g., a wafer 100, that includes a p-layer101 and an n-layer 102, which will serve as an emitter. For example, apreliminary process for preparing the rear electrode 210 may beperformed by printing the paste composition on the rear surface of thewafer 100 and drying the printed paste at about 200° C. to about 400° C.for about 10 to 60 seconds. Further, a preliminary process for preparingthe front electrode may be performed by printing the paste on the frontsurface of the wafer and drying the printed paste. Then, the frontelectrode 230 and the rear electrode 210 may be formed by baking thewafer at about 400° C. to about 950° C., e.g., at about 850° C. to about950° C., for about 30 to about 50 seconds.

The following Examples and Comparative Examples are provided in order tohighlight characteristics of one or more embodiments, but it will beunderstood that the Examples and Comparative Examples are not to beconstrued as limiting the scope of the embodiments, nor are theComparative Examples to be construed as being outside the scope of theembodiments. Further, it will be understood that the embodiments are notlimited to the particular details described in the Examples andComparative Examples.

EXAMPLES

Specifications of components used in the following Examples andComparative Examples were as follows.

(A) Conductive powder: Spherical silver powder (Dowa Hightech Co., Ltd.,AG-4-8) having an average particle size (D50) of 2 μm.

(B) Glass fit: The following components were mixed in amounts as listedin Table 1 (unit: wt %) and melted at 1200° C., followed by quenching to25° C. The resultant was pulverized using a disk mill, thereby preparinga glass frit having an average particle size D50 of 2 μm.

TABLE 1 PbO SiO₂ TeO₂ Bi₂O₃ P₂O₅ Li₂O ZnO NiO WO₃ Co₂O₃ Example 1 40.005.00 22.00 20.00 0.00 3.00 0.00 10.00 0.00 0.00 Example 2 40.00 5.0022.00 20.00 0.00 3.00 0.00 0.00 10.00 0.00 Example 3 40.00 5.00 22.0020.00 0.00 3.00 0.00 0.00 0.00 10.00 Example 4 45.00 0.00 27.00 20.001.00 2.00 0.00 5.00 0.00 0.00 Example 5 45.00 0.00 27.00 20.00 1.00 2.000.00 0.00 5.00 0.00 Example 6 45.00 0.00 27.00 20.00 1.00 2.00 0.00 0.000.00 5.00 Example 7 15.00 0.00 30.00 40.00 0.00 0.00 5.00 0.00 10.000.00 Example 8 40.00 5.00 30.00 10.00 0.00 0.00 5.00 0.00 10.00 0.00Example 9 15.00 0.00 30.00 40.00 0.00 0.00 5.00 10.00 0.00 0.00 Example10 15.00 0.00 30.00 40.00 0.00 0.00 5.00 0.00 0.00 10.00 Example 1110.00 0.00 30.00 40.00 0.00 0.00 5.00 0.00 15.00 0.00 Example 12 10.000.00 30.00 40.00 0.00 0.00 15.00 0.00 5.00 0.00 Comparative 65.00 10.0022.00 0.00 0.00 3.00 0.00 0.00 0.00 0.00 Example 1 Comparative 40.000.00 57.00 0.00 0.00 3.00 0.00 0.00 0.00 0.00 Example 2 Comparative40.00 5.00 22.00 20.00 0.00 3.00 10.00 0.00 0.00 0.00 Example 3

(C) Organic vehicle: Ethyl cellulose (Dow Chemical Company, STD4) andbutyl carbitol.

Examples and Comparative Examples: Preparation of Paste Composition

11 g of organic vehicle obtained by mixing 86 g of silver powder, 1 g ofethyl cellosolve, and 10 g of butyl carbitol was added to 3 g of theglass frit prepared as above, followed by mixing and kneading in a3-roll kneader, thereby preparing a paste composition for solar cellelectrodes. The paste composition was then deposited on a screen printplate by rolling a scraper thereon. The paste composition was printed ona polycrystalline wafer having an average surface resistance of 80 Ωwhile squeezing the paste composition to an image area of the screenprinting plate. The printed wafer was subjected to baking in a BTUbaking furnace at a 6-zone temperature of 950° C. and a belt speed of250 rpm. After baking, solar cell efficiency (%) was measured using aPASSAN cell tester.

TABLE 2 Efficiency (%) Example 1 16.35 Example 2 16.88 Example 3 16.15Example 4 16.38 Example 5 16.55 Example 6 16.64 Example 7 16.06 Example8 16.26 Example 9 16.32 Example 10 16.27 Example 11 16.22 Example 1216.29 Comparative Example 1 10.89 Comparative Example 2 14.5 ComparativeExample 3 12.78

As shown in Table 2, the electrodes prepared from the paste compositionsof the Examples had high solar cell efficiency. On the other hand, theelectrodes prepared using the paste compositions including the glassfrit free from the transition metal oxide component (i.e., free of thetransition metal oxide having a melting point of about 1300° C. or more)had considerably lower solar cell efficiency.

By way of summation and review, electrodes of a solar cell may be formedon a wafer by applying, patterning, and baking an electrode paste forelectrodes. Continuous reduction of thickness of an emitter forimprovement of solar cell efficiency may cause shunting, which maydeteriorate solar cell performance. In addition, a solar cell size maybe increased in area to achieve high efficiency. In this case, however,efficiency may deteriorate due to increase in contact resistance of thesolar cell. Further, with increasing use of wafers having varioussurface resistances, a temperature range for baking may be widened andthus there is an increasing need for electrode pastes capable ofsecuring thermal stability in a wide sintering temperature range.Therefore, there is a need for development of glass frits and electrodepastes capable of securing p-n junction stability while improving solarcell efficiency by minimizing adverse influence on the p-n junctiongiven varying surface resistances.

In fabrication of a crystalline silicon-based solar cell, the thicknessof the p-n junction may vary upon surface treatment of a siliconsubstrate, or due to unevenness of an anti-reflective layer, emitterlayer, and the like. Thickness deviation of the p-n junction due tovariation of each wafer lot may cause an increase in variation of solarcell efficiency and may deteriorate solar cell efficiency. Thus, a lowdegree of efficiency variation may correlate with high stability of thep-n junction. A glass frit that includes PbO, B₂O₃, and SiO₂ may providea narrowed efficiency deviation in a certain composition range. A glassfrit that contains 25 mol % or more of TeO₂ may provide low contactresistance and high p-n junction quality. A glass frit composed ofPb—Te—B may realize low contact resistance. However, these techniquesmay be limited in regards to reduction of contact resistance in highsurface resistance.

As described above, embodiments relate to a paste composition for solarcell electrodes, which may improve solar cell efficiency by minimizingadverse influence on a p-n junction given varying surface resistanceswhile reducing contact resistance, and electrodes fabricated using thesame. Embodiments may provide a paste composition for electrodes thathelps minimize adverse influence on a p-n junction in high surfaceresistance while reducing contact resistance, thereby realizing highefficiency electrodes. Embodiments may provide a solar cell includingelectrodes fabricated using the paste composition for electrodes.Embodiments may provide paste compositions for solar cell electrodes,which minimize or avoid adverse influence on a p-n junction givenvarying surface resistances, and solar cell electrodes fabricated usingthe same.

Example embodiments have been disclosed herein, and although specificterms are employed, they are used and are to be interpreted in a genericand descriptive sense only and not for purpose of limitation. In someinstances, as would be apparent to one of ordinary skill in the art asof the filing of the present application, features, characteristics,and/or elements described in connection with a particular embodiment maybe used singly or in combination with features, characteristics, and/orelements described in connection with other embodiments unless otherwisespecifically indicated. Accordingly, it will be understood by those ofskill in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. A paste composition for a solar cell electrode,the composition comprising: an organic vehicle; a conductive powder; anda glass frit, the glass frit including: TeO₂; and a transition metaloxide component, the transitional metal oxide component including one ormore of a transition metal oxide having a melting point of about 1300°C. or more, wherein a weight ratio of the TeO₂ to the transition metaloxide component ranges from about 2:1 to about 6:1.
 2. The compositionas claimed in claim 1, wherein the glass frit includes about 1 wt% toabout 15 wt% of the transition metal oxide component.
 3. The compositionas claimed in claim 1, wherein the transition metal oxide includes atleast one of NiO, WO₃ and Co₂O₃.
 4. The composition as claimed in claim1, wherein the glass frit includes about 15 wt% to about 70 wt% of theTeO₂.
 5. The composition as claimed in claim 1, wherein the TeO₂ and thetransition metal oxide component are present in a total amount of about16 wt% to about 75 wt% in the glass frit.
 6. The composition as claimedin claim 1, wherein the glass frit further includes about 5 wt% to about35 wt% of Bi₂O₃ and about 10 wt% to about 50 wt% of PbO.
 7. Thecomposition as claimed in claim 1, wherein the glass fit furtherincludes about 1 wt% to about 20 wt% of ZnO.
 8. The composition asclaimed in claim 1, wherein the glass frit further includes one or moreof Al₂O₃, ZrO₂, P₂O₅, SiO₂, Na₂O, B₂O₃, Ta₂O₅, Fe₂O₃, Cr₂O₃, Li₂O,Li₂CO₃, MgO, or MnO₂.
 9. The composition as claimed in claim 1, whereinthe conductive powder includes one or more of silver, gold, palladium,platinum, copper, chromium, cobalt, aluminum, tin, lead, zinc, iron,iridium, osmium, rhodium, tungsten, molybdenum, nickel, or indium tinoxide powder.
 10. The composition as claimed in claim 1, wherein theorganic vehicle includes one or more of a binder or a solvent.
 11. Thecomposition as claimed in claim 1, comprising: about 5 wt% to about 30wt% of the organic vehicle, about 60 wt% to about 90 wt% of theconductive powder, and about 1 wt% to about 10 wt% of the glass frit.12. The composition as claimed in claim 1, further comprising one ormore of a dispersant, a thixotropic agent, a plasticizer, a viscositystabilizer, an anti-foaming agent, a pigment, a UV stabilizer, anantioxidant, or a coupling agent.
 13. A solar cell electrode fabricatedusing the composition as claimed in claim
 1. 14. A method of fabricatinga solar cell having a solar cell electrode, the method comprising:printing the composition as claimed in claim 1 on a substrate; andbaking the substrate having the composition thereon to form the solarcell electrode.